Gypsum wallboard

ABSTRACT

Gypsum wallboard can be made lighter and less dense, without sacrificing strength, by adding to the gypsum slurry used in making the board a styrene butadiene polymer latex substantially stable against divalent ions in which the styrene butadiene polymer includes at least 0.25 wt. % of an ionic monomer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation filing of U.S. application Ser. No.12/080,656, filed Apr. 4, 2008, now U.S. Pat. No. 7,879,965, which is acontinuation filing of U.S. application Ser. No. 10/878,862, filed Jun.28, 2004, now abandoned, which is a continuation filing of U.S.application Ser. No. 09/464,174, filed Dec. 16, 1999, now U.S. Pat. No.6,755,907, which is a continuation-in-part filing of U.S. applicationSer. No. 09/237,512, filed Jan. 26, 1999, now U.S. Pat. No. 6,184,287.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to improved gypsum wallboard and tomaterials and processes for making such products.

2. Background

Gypsum wallboard is conventionally made by depositing an aqueous slurryof calcined gypsum (“gypsum slurry”) between large sheets of paper orother material and allowing the slurry to dry. Calcined gypsum iscomposed of calcium sulfate hemihydrate (CaSO₄.½H₂O) which rehydrates togypsum (CaSO₄.2H₂O) during the drying process. See Kirk Othmer,Encyclopedia of Chemical Technology, Second edition, 1970, Vol. 21,Pages 621-624, the disclosure of which is also incorporated herein byreference.

In order to achieve sufficient strength, traditional, commercialwallboard has been made with a density of about 1700 pounds (˜772 kg.)per thousand square feet of ½ inch thick board. Although it would bedesirable to reduce this density and hence overall board weight,previous attempts have met with limited success, primarily due to lossof strength.

U.S. Pat. No. 5,879,825 to Burke et al., the disclosure of which is alsoincorporated herein by reference, describes an approach for reducinggypsum wallboard density without sacrificing strength by including inthe gypsum core an acrylic latex having a particular combination ofproperties. Acrylic latexes are expensive, and therefore commerciallyunattractive. Therefore, it is desirable to develop an alternate andless expensive approach to accomplishing this objective.

SUMMARY OF THE INVENTION

In accordance with the present invention, it has been discovered thatcertain styrene butadiene latexes, modified to be substantially stableagainst divalent ions, can also reduce gypsum wallboard density withoutsacrificing strength. Because these latexes are generally less expensiveto manufacture than acrylic latexes, it has also been found thatcommercial use of these latexes is feasible.

Accordingly, the present invention provides a new composition for makinggypsum products comprising water, calcium sulfate hemihydrate and astyrene butadiene polymer latex substantially stable against divalentions in which the styrene butadiene polymer includes at least 0.25 wt. %of an ionic monomer. In addition, the present invention also provides anew process for making gypsum wallboard from this composition as well asthe wallboard so made. Preferably, the styrene butadiene polymer of thelatex used in the present invention includes copolymerized sodium2-acrylamido-2-methyl propanesulfonic acid salt, known industrially as“sodium AMPS.”

DETAILED DESCRIPTION

In accordance with the present invention, gypsum wall board can be madelighter in weight without sacrificing strength by including in thegypsum slurry used to make the board a styrene butadiene polymer latexsubstantially stable against divalent ions in which the styrenebutadiene polymer includes at least 0.25 wt. % of an ionic monomer.

The styrene butadiene latexes used in accordance with the presentinvention are substantially stable against divalent ions. By“substantially stable against divalent ions” is meant that a latex willexhibit no significant coagulation or flocculation when 10 ml(milliliters) of a 2 wt. % calcium chloride aqueous solution is slowlyadded to 50 ml of the latex. By slowly added is meant that the calciumchloride solution is added to 50 ml of the latex with stirring over aperiod of time between 5 and 30 seconds.

The amount of styrene and butadiene in the polymers of these latexes canvary widely. For example, these polymers may contain 4 to 60 wt. %butadiene, more normally 7 to 40 wt. % butadiene and especially 10 to 30wt. % butadiene. In addition, they may contain 20 to 95 wt. % styrene,more normally 45 to 90 wt. % styrene aid especially 65 to 85 wt. %styrene. Moreover, in these polymers, the ratio of styrene to butadieneis typically in the range of 10/1 to 1/1, more usually 7/1 to 1.5/1, andeven more typically 6/1 to 2/1.

In addition to styrene and butadiene, the styrene butadiene polymers ofthe present invention also include an ionic monomer. By “ionic monomer”is meant a monomer which addition polymerizes to form a homopolymerwhich is water soluble when having a molecular weight of 5000. In otherwords, if a 5000 molecular weight homopolymer formed by additionpolymerizing a monomer is water soluble, that monomer is “ionic” in thecontext of this invention. Examples of suitable ionic monomers are2-acrylamido-2-methyl propanesulfonic acid salt, styrene sulfate salt,styrene sulfonate salt, allyl sulfonate salt, 3-sulfopropyl acrylatesalt, 3-sulfopropyl methacrylate salt, 2-sulfoethyl acrylate salt,2-sulfoethyl methacrylate salt, maleic acid, itaconic acid and salts ofmaleic acid and itaconic acid. The cations of these salt are normallysodium, potassium or ammonium, more typically sodium or potassium.2-acrylamido-2-methyl propanesulfonic acid salt is the preferred ionicmonomer, with sodium 2-acrylamido-2-methyl propanesulfonic acid saltbeing especially preferred. 2-acrylamido-2-methyl propanesulfonic acidis known in industry as “AMPS,” which is a trademark of The LubrizolCompany.

The amount of ionic monomer in the styrene butadiene polymers of theinvention can vary widely. As little as about 0.25 wt. % to as much asabout 20 wt. %, based on the weight of the polymer, are effective.Typically, the polymers will contain about 0.5 to 10, more often about 1to 5 wt. %, ionic monomer based on the weight of the polymer.

In addition to styrene, butadiene and the ionic monomer, the polymers ofthe invention may also include 0.25 to 20 wt. % of hydroxyethylacrylate, hydroxyethyl methacrylate, acrylonitrile, methacrylonitrile,acrylamide and/or methacrylamide. These “hydrophilic adjunct comonomers”have been found to enhance the effect of the ionic monomers in that theoverall stability against divalent ions exhibited by a polymer includingan ionic monomer as well as a hydrophilic adjunct comonomer is greaterthan would have been predicted by the rule of mixtures. A hydrophilicadjunct comonomer content of 0.5 to 10 wt. %, or even 1 to 4 wt. %, ismore typical.

In addition to the foregoing monomers, the styrene butadiene polymers ofthe present invention may also include other addition monomers. Examplesare isoprene, chloroprene, alpha-methylstyrene, 4-methylstyrene,4-tert-butylstyrene, 4-ethylstyrene, divinylbenzene, vinylidenechloride, 2-vinylpyridene, 4-vinylpyridene and especially acrylic acid,methacrylic acid and their derivatives such as metal and ammonium salts,substituted and unsubstituted amides (other than acrylamide which is anhydrophilic adjunct comonomer), nitriles, and C₁ to C₁₂ esters. In suchcases, the polymer should contain no more than about 30 wt. %, moretypically no more than 15 wt. %, other addition monomer.

Styrene butadiene polymer latexes are typically made by aqueous emulsionpolymerization. In carrying out such processes, the monomers forming thepolymer are emulsified in water using suitable surfactants, usuallyanionic or non-ionic. Other ingredients such as free-radical initiators,chelating agents, chain transfer agents, biocides, defoamers andantioxidants can also be added. Once the free-radical initiator isactivated, the monomers polymerize together producing the productpolymer. As is well known, the arrangement of multiple monomers in aproduct polymer can be determined, at least to some degree, bycontrolling the manner which the monomers are added to the system. Ifall the monomers are added at the same time, the product polymer willhave a more random distribution of monomers. If added in stages, thepolymer will have a more ordered distribution of monomers.

A typical polymer latex produced by emulsion polymerization containsenough surfactants and other ingredients to prevent the product polymerfrom separating out from the water phase upon standing. However, thesesurfactants and other ingredients are usually insufficient to preventcoagulation or flocculation of the polymer if the latex is contaminatedwith significant amounts of divalent or trivalent ions. Therefore, suchlatexes can be expected to coagulate or flocculate prematurely ifcontacted with gypsum slurries used in the manufacture of gypsumwallboard, since such slurries contain significant concentrations ofcalcium ions, which are divalent.

In order to avoid this problem, additional surfactants cap beincorporated into the latexes to keep them stable against calcium ions.However, gypsum slurries already contain significant concentrations ofparticular types of surfactants and other ingredients to enable theslurries to be frothed (foamed) during manufacture. Accordingly, it isdesirable to avoid adding still additional surfactants to these systems,since different surfactant packages can interact with one another andthereby become ineffective.

In accordance with the present invention, therefore, the styrenebutadiene latex includes a significant amount of a monomer, the ionicmonomer, which imparts its own surface active properties to the polymer.As a result, additional surfactants for imparting calcium ion stabilityto a latex of the polymer can be reduced or even eliminated entirely.Therefore, when such latexes are added to gypsum slurries in themanufacture of gypsum wallboard, problems occurring from mixingincompatible surfactant packages can be avoided.

Styrene butadiene latexes which are especially useful in accordance withthe present invention are described in commonly-assigned applicationSer. No. 09/237,512, filed Jan. 26, 1999, now U.S. Pat. No. 6,184,287,the disclosure of which is incorporated herein by reference. In general,these latexes are formed by emulsion polymerization of styrene andbutadiene in the presence of an in situ seed polymer composed ofpolymerized styrene and an AMPS salt, preferably Na-AMPS. Normally, theseed polymer is made by emulsion polymerization of styrene and Na-AMPSonly, although butadiene may be included as an additional comonomer ifdesired. These latexes have been designed for mixing with cement usedfor cementing oil wells and are particularly stable against divalentions. The conditions encountered in gypsum slurries are less severe thanthose in oil well cementing, and so less ionic monomer may be acceptablewhen these polymers are used in the present invention as compared to oilwell cementing applications.

Styrene butadiene latexes of particular utility in accordance with thepresent invention are formed by emulsion polymerizing the monomersidentified in the following Table 1 in accordance with the generalprocedure described in the above-noted U.S. Pat. No. 6,184,287(application Ser. No. 09/237,512, filed Jan. 26, 1999):

TABLE 1 Components of Styrene Butadiene Polymers, wt. % Example NaAMPSStyrene Butadiene HEA¹ Acrylo² Results 1 2.5 77.5 15 3 2 2.5 67.5 15 310 3 5.5 67.2 26 1.3 ¹Hydroxyethyl acrylate ²Acrylonitrile

The amount of styrene butadiene latex that should be incorporated into agypsum slurry in accordance with the present invention can vary widely,and essentially any amount can be used. From a practical standpoint, theamount of latex should be enough so that a noticeable decrease indensity of product gypsum wallboard can be achieved without sacrificingstrength but not so much that the product wallboard product becomeseconomically unattractive. In general, this means that the amount oflatex added should be enough so that the styrene butadiene polymer ispresent in the product composition is about 0.1 to 10 wt. %, based onthe weight of calcium sulfate hemihydrate in the composition. Moretypically, the amount of styrene butadiene polymer in the composition is0.25 to 5 wt. %, and especially 0.5 to 1.5 wt. %, based on the weight ofcalcium sulfate hemihydrate in the composition.

Gypsum slurries for manufacture of wallboard typically contain variousadditional ingredients, as well known to those skilled in the art.Examples of such ingredients are accelerators, starch, retarders, paperpulp and so forth. See the above-noted Burke et al. patent, U.S. Pat.No. 5,879,825, especially Table I. Such components can also be includedin the compositions produced in accordance with the present invention.

The gypsum-containing compositions of the present invention are used inthe same way as conventional gypsum slurries to manufacture gypsumwallboard product. That is, they are deposited between large sheets ofpaper or other material and allowed the dry whereby the calcium sulfatehemihydrate in the system rehydrates into the dihydrate, i.e. gypsum,thereby forming the completed wallboard product. In commercial practice,the process is carried out in high volume using machines havingtraveling webs which rapidly move the inoipiently-formed product throughovens under precisely controlled heating conditions for removing exactlythe right amount of water. In this environment, it is desirable that theamount of water in the starting gypsum slurry be controlled so that thewallboard product is dried to the right amount when it leaves the oven.

To this end, conventional gypsum slurries for making wallboard typicallycontain about 40 to 60 wt. %, more typically about 48 to 55 wt. %calcium sulfate hemihydrate and less than 60 wt. %, more typically lessthan 50 wt. % water, based on the weight of the composition. See Table Iof the above-noted Burke et al. patent. The gypsum slurries of thepresent invention may also contain the same amounts of calcium sulfatehemihydrate and water, especially when intended for use in making gypsumwallboard in modern high speed equipment.

In this connection, it should be appreciated mat the amount of waterwhich a styrene butadiene styrene butadiene latex adds to a gypsumslurry in accordance with the present invention is essentially trivialwhen making wallboard product under normal practice. This is because theamount of latex added will typically be small, e.g. 5 wt. % or less, andthe amount of water in this latex will usually be less than 50 wt. %.Also, less gypsum slurry is needed to make a wallboard product of agiven dimension, since its density is less, and hence less water derivedfrom the gypsum slurry is present in the inventive gypsum slurries inthe first place. This means that the net effect of including a styrenebutadiene latex in a gypsum slurry in accordance with the presentinvention may actually be to reduce the overall water content of theslurry by a slight amount for a wallboard product of a given dimension.In any event, those skilled in the art can readily determine by routineexperimentation the precise amount of calcium sulfate hemihydrate andwater to include in a particular embodiment of the inventive gypsumslurries in order that it can be used without problem in making gypsumwallboard in modern high speed equipment.

It should also be appreciated that the gypsum slurries of the presentinvention can be used in applications other than in making gypsumwallboard. For example, the inventive gypsum slurries can also be usedin making molding plasters. In these applications, more or less waterthan indicated above can be included in the composition depending on theparticular application desired. Indeed, the only real upper limit on thewater content of the inventive gypsum slurry is that too much water maymake its viscosity too low for practical application or may cause waterto separate out. Similarly, the only real lower limit on the watercontent is stoichiometric—that is, enough water should be present toallow substantially complete hydration of the calcium sulfatehemihydrate to the dihydrate form. Within these broad limits, thoseskilled in the art can readily determine by routine experimentation theprecise amount of water to use in a particular application.

Although only a few embodiments of the present invention have beendescribed above, it should be appreciated that many modifications can bemade without departing from the spirit and scope of the invention. Allsuch modifications are intended to be included within the scope of thepresent invention, which is to be limited only by the following claims.

I claim:
 1. A latex composition, for use in making gypsum wall board,including a polymer having a styrene block and comprising: about 4 to 70wt. % butadiene; about 20 to about 95 wt. % styrene; and at least 0.25wt. % of an ionic monomer comprised of 2-acrylamido-2-methylpropanesulfonic acid salt; wherein said 2-acrylamido-2-methylpropanesulfonic acid salt is present predominantly within the styreneblock of said polymer.
 2. The latex according to claim 1, wherein thelatex is substantially stable against divalent ions.
 3. The latexaccording to claim 1, wherein the latex is substantially free ofsurfactants that impart stability against divalent ions.
 4. The latexaccording to claim 1, wherein the ratio of styrene to butadiene in thepolymer is in the range of from about 10:1 to 1:1.
 5. The latexaccording to claim 1, wherein the 2-acrylamido-2-methyl propanesulfonicacid salt is sodium 2-acrylamido-2-methyl propanesulfonic acid.
 6. Thelatex according to claim 1, wherein said 2-acrylamido-2-methylpropanesulfonic acid salt is present only within the styrene block ofsaid polymer.